The present invention relates generally to a system for transmitting data from a transmitter comprising at least two transmitting antennas, an encoder following an error correcting code structure and a transmit beamforming technique to a receiver comprising at least two receiving antennas and a decoder defined according to the error correcting code structure over a transmit beamformed MIMO channel.
In the following, a channel resource means a frequency bandwidth, a time interval and possibly several spatial dimensions brought by transmitting and receiving antennas placed in different spatial positions.
Signals transmitted over wireless channels undergo severe degradations due to channel variations such as fading and shadowing, which allow to consider the channel as a random variable. In the following, are considered slow channel variations with respect to the time needed for transmission of the information words, but the channel realization is supposed to have changed between two transmissions of information words. One major way to combat the so-called quasi-static fading is to provide diversity in either time, frequency or space.
The channel diversity order is defined as the number of independent fading random variables observed in the channel resource used for transmission. The transmission/reception scheme is able to collect a given amount of diversity, called diversity order of the system, upper bounded by the channel diversity order, also called full diversity order.
When an information word is not correctly estimated by the receiver, an error event occurs. The main parameter allowing to compute the probability of error associated to a given error event is the Euclidean distance between the noiseless received signal associated to the transmitted information word and the reconstructed noiseless received signal associated to the decoded information word. The diversity order of the error event is defined as the number of independent random variables involved in the Euclidean distance associated to the error event. Finally, the system diversity order is equal to the minimal diversity order of all possible error events or equivalently all possible pairs of information words.
Telecommunication systems in which a plurality of antennas are used at a receiver end and/or at a transmitter end of a wireless link are called Multiple Input Multiple Output systems (further referred to as MIMO systems). MIMO systems have been shown to offer large transmission capacities compared to those offered by single antenna systems. In particular, MIMO capacity increases with the number of transmitting or receiving antennas, whichever the smallest, for a given Signal-to-Noise Ratio and under favourable uncorrelated channel conditions. MIMO techniques are thus likely to be used in future wireless systems intended to provide large spectral efficiencies or, alternatively, reduce the transmitting power required for obtaining a spectral efficiency equivalent to that which is obtained in current telecommunication systems. Such MIMO techniques will very likely be combined with multi-carrier modulation techniques like OFDM, which allow to consider MIMO channel models with no inter-symbol interference.
The transmitter of MIMO systems comprises a digital modulator converting the coded bits stream into spatial streams of modulation symbols, the inputs of said digital modulator are coded bits, and the output of said digital modulator is vectors of Ns≦min(Nt, Nr) modulation symbols. The system is then said to transmit Ns spatial streams on the transmit MIMO channel.
An eigenvector transmit beamforming scheme can be used for improving the performance of MIMO channels. The matrix of the transmit beamformed MIMO channel is the concatenation of a Ns×Nt beamforming matrix and the Nt×Nr matrix H of the channel. The Ns×Nt beamforming matrix is derived from the eigenvectors associated to the Ns best eigenvalues of the Nt×Nr matrix of the channel. The transmit beamforming technique then needs a partial knowledge of the matrix H at the transmitter to apply a pre-filtering which allows a low-complexity optimal detection at the receiver. For example, the partial knowledge is a quantized version of the channel estimation processed at the receiver and fed back from the receiver to the transmitter.
The eigenvector transmit beamforming scheme may reach the full diversity order under the assumption of a single transmitted spatial stream. When multiple spatial streams are transmitted with the aim of increasing the data rate of the scheme, the diversity order of the scheme is drastically reduced.